Optical transceivers must be designed to transmit signals with sufficient power to meet the transmission distance and performance requirements of the system. At the same time, standard bodies such as the International Electrotechnical Commission (IEC) and the Center for Devices and Radiological Health (CDRH) have imposed limits on the magnitude of optical power that can be transmitted in a link between two transceivers, to protect the human eye in case of an open fiber. An open fiber is any discontinuity in the signal path that causes the signal light to escape into the open, e.g. a physical break in the optical fiber or an unplugged connector. Usually, an open fiber is the result of an unplugged connector. Unfortunately, these safety constraints limit the usefulness of multi-channel optical transceivers, which employ techniques such as multi-channel parallel transmission or wavelength division multiplexing (WDM).
Multiple fibers in a single ribbon cable are used to transmit data in multi-channel parallel transmission. The greater the number of fiber-optic cables used, the greater the amount of data can be transmitted. The fiber-optic cables are normally packed closely together in the ribbon cable, each fiber-optic cable constituting a single channel for signal transmission. Since the fiber-optic cables are closely packed, the optical power emitted from an end of an unconnected ribbon cable can be the optical power of several fiber-optic cables combined.
WDM combines multiple signals of different wavelengths into a single fiber for transmission. Using WDM, several signals can be transmitted at the same time along a single fiber, thus allowing a single fiber to transmit multiple channels of data. The optical power of the combined, transmitted signal is the sum of the power of each constituent signal.
These techniques enable transceivers to transmit at higher data rates by using multiple channels, but the total power transmitted still cannot exceed the eye-safety limits. The safeguards require that the combined power of the multiple channels be kept below the eye-safety limit. For example, if there are N channels in a multi-channel transceiver, and the power distribution between each channel is identical, each channel can operate at approximately only 1/Nth of the maximum eye safe power level. In this way, the combined power of the N channels does not exceed the maximum eye-safety level. However, reducing the power of the signals in each channel also reduces the distance the signals can travel without losing integrity. An optical network using reduced power signals will require more repeaters or higher sensitivity receivers to compensate for a weaker signal.
Open Fiber Control (OFC) is one way to circumvent the eye-safety constraints without endangering the human eye. OFC is a method for enabling and disabling laser transmissions in optical transceivers. When two transceivers are properly connected, each channel in a multiple channel application can be operated at eye-safety limits. Although the total optical power transmitted exceeds the eye-safety limits, there is no possibility of injury since no light escapes into the open. When an open fiber is detected, the transmissions must be either reduced to safe levels or shut down completely.
OFC may be implemented using software to control the optical transceivers. When an open fiber is detected, the software immediately shuts down transmissions from the transceiver. To re-establish the link, the software instructs each transceiver to periodically send out coded signal pulses in a handshake routine. However, it is difficult to synchronize and decode these signals, especially given varying link lengths and longer distances between transceivers. Therefore, OFC implemented in software is not widely adopted despite its advantages.
Recently, however, bandwidth requirements have increased to the point where multiple channel techniques like the ones described earlier are needed to accommodate the higher data rates. 1 Gigabit per second (Gbps) and 10 Gbps data rates are common now, and are sure to increase in the future. However, without using complicated OFC software routines, optical transceivers cannot currently transmit over multiple channels at high power without exceeding eye-safe levels.